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    SubjectRe: [PATCH] rcu: avoid checking for constant
    On Thu, Jan 12, 2012 at 10:25:23AM +0100, Jan Engelhardt wrote:
    > rcu: avoid checking for constant
    >
    > When compiling kernel or module code with -O0, "offset" is no longer
    > considered a constant, and therefore always triggers the build error
    > that BUILD_BUG_ON is defined to yield.
    >
    > What is the rationale between the forced constant check,
    > introduced in 9ab1544eb4196ca8d05c433b2eb56f74496b1ee3?

    This question shouldn't live in the commit message. Please replace it
    with an explanation of the change instead.

    > Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
    > ---
    > include/linux/rcupdate.h | 22 ++++------------------
    > 1 files changed, 4 insertions(+), 18 deletions(-)
    >
    > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    > index 2cf4226..5e7286d 100644
    > --- a/include/linux/rcupdate.h
    > +++ b/include/linux/rcupdate.h
    > @@ -795,24 +795,6 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
    > #define RCU_INIT_POINTER(p, v) \
    > p = (typeof(*v) __force __rcu *)(v)
    >
    > -static __always_inline bool __is_kfree_rcu_offset(unsigned long offset)
    > -{
    > - return offset < 4096;
    > -}
    > -
    > -static __always_inline
    > -void __kfree_rcu(struct rcu_head *head, unsigned long offset)
    > -{
    > - typedef void (*rcu_callback)(struct rcu_head *);
    > -
    > - BUILD_BUG_ON(!__builtin_constant_p(offset));
    > -
    > - /* See the kfree_rcu() header comment. */
    > - BUILD_BUG_ON(!__is_kfree_rcu_offset(offset));
    > -
    > - call_rcu(head, (rcu_callback)offset);
    > -}
    > -
    > /**
    > * kfree_rcu() - kfree an object after a grace period.
    > * @ptr: pointer to kfree
    > @@ -836,6 +818,10 @@ void __kfree_rcu(struct rcu_head *head, unsigned long offset)
    > * Note that the allowable offset might decrease in the future, for example,
    > * to allow something like kmem_cache_free_rcu().
    > */
    > +#define __kfree_rcu(head, offset) \
    > + call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset) + \
    > + BUILD_BUG_ON_ZERO((offset) >= 4096))
    > +

    I had to stare at this for a while, and look up the definition of
    BUILD_BUG_ON_ZERO. Naturally I assumed that BUILD_BUG_ON_ZERO(arg)
    meant BUILT_BUG_ON((arg) == 0), which would have made the logic
    backwards here. However, per the definition it just provides a
    zero-returning version of BUILD_BUG_ON. Ow.

    In any case, __kfree_rcu has void return type, so how about just using
    do { ... } while(0) and BUILD_BUG_ON instead? That seems significantly
    clearer.

    Apart from that, I can live with this, though it seems horribly
    backwards to replace an inline with a macro rather than the other way
    around.

    - Josh Triplett


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